1. Field of the Invention
The present invention relates to a semiconductor device such as an electronic element, an optical element, and integrated circuit thereof which performs functions of light-emission, sensing, or the like, and also relates to a process for producing the semiconductor device. In particular, the present invention relates to a semiconductor device having a porous region and to a process for producing the device.
2. Related Background Art
Porous materials are attracting attention in recent years as a novel functional material for constituting an active region of an element.
The porous structure of a semiconductor crystal of Group IV, for example, were primarily used in the past as structural members which are not related to the electronic and optical properties of the material and uses such as formation of SOI (Silicon On Insulator) structure utilizing capability of high speed oxidation (see, for example, T. Unagami and M. Seki: J. Electrochem. Soc., 125, 1339 (1978)).
In recent years, the porous material is applied to an active region material of a gas sensor by utilizing the large surface area resulting from the porosity (see, for example, A. Motohashi, et al.: Oyo Butsuri Gakkai Gakujutsu Koenkai (Shuki) Yokoshu (Preprint of Autumnal Meeting of Society of Applied Physics), 29p-ZB-9 (1986)); and light-emission phenomenon was discovered which is caused by a structure sufficiently fine for imprisonment of carrier quantum (see L. T. Canham, Appl. Phys. Lett., 56, 1046 (1990)). Since then, studies are actively being made on functional application of porous material to light-emitting element, and other applications. The functional application of the porous material will possibly become an attractive novel technique if several practical problems are solved, because the porous structure can be formed readily by treating the base material.
The most serious of the problems in realization of the functional application of the porous material is the lack of stability of structure of the material.
The porous material, as understood from the name, inherently has many voids in the space where the material is to be filled. Therefore, the structural strength is unavoidably low in comparison with the base material. Further the absolute value of the strength naturally depends on the size, shape, density, and so forth of the remaining structure. A material of finer porous structure is generally more brittle than the one having a coarser structure. Accordingly, a coarse porous material is advantageous as far as desired functions are performed with the coarse structure. However, in application fields in which finer structure is required, the structural instability becomes a serious problem.
Referring again to the example of the aforementioned semiconductor crystal of Group IV, a finer structure is desired to obtain a larger surface area for higher sensitivity of a gas sensor; and visible light-emission is not achievable without a superfine structure of finer than several nanometers. In the latter application, to obtain shorter wavelength of emitted light, much more finer structure is required.
Practically, however, such a superfine porous material is extremely brittle, and will collapse immediately on touching. Therefore, it is extremely difficult to practicalize the instable material with a definite element size.
Another problem in functional application of a porous material is the difficulty of signal transmission between the porous active region and the outside.
The active element needs to transmit or receive signal or energy to or from the porous region. However, the porous structure or the composition of the intended porous material is frequently not suitable for the transmittance function. For example, the aforementioned light-emitting element involves the problem that an electrode formed for injecting carriers into the porous active region is insufficient in structural strength and further does not give sufficient injection efficiency because of excessively high contact resistance.
Generally, unsolved problems exist not only in transmission of signals and energies but also in simultaneous practice of a plurality of functions.